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SubjectRe: [Stable-review] [24/28] USB: xhci - fix unsafe macro definitions
On Wed, Apr 20, 2011 at 03:02:04AM +0100, Ben Hutchings wrote:
> On Tue, 2011-04-19 at 13:31 -0700, Greg KH wrote:
> > 2.6.32-longterm review patch. If anyone has any objections, please let us know.
> >
> > ------------------
> >
> > From: Dmitry Torokhov <dtor@vmware.com>
> >
> > commit 5a6c2f3ff039154872ce597952f8b8900ea0d732 upstream.
> >
> > Macro arguments used in expressions need to be enclosed in parenthesis
> > to avoid unpleasant surprises.
>
> Do you know of any specific uses of these macros where the missing
> parentheses caused 'unpleasant surprises'?

In my opinion, this type of fix should be backported even if the current
code does not appear to be at risk, otherwise a later fix in the kernel
could cause a serious regression when backported to -stable. For instance,
if we later have to backport this patch (cut'n'pasted) :

--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -1033,7 +1033,7 @@ static inline unsigned int xhci_get_endpoint_interval(struct usb_device *udev,
default:
BUG();
}
- return EP_INTERVAL(interval);
+ return EP_INTERVAL(interval + 1);
}

How can you guess that while works in mainline, it breaks -stable ?

As a user, I'd rather have known valid code in -stable and -longterm at the
risk of an occasional *temporary* regression than longterm unexplainable
regressions due to diverging code causing unexpected issues with backported
fixes.
Regards,
Willy



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